1,721,155 research outputs found
Mean bond-length variation in crystal structures: A bond-valence approach
No full textThe distortion theorem of the bond-valence theory predicts that the mean bond length D increases with increasing deviation of the individual bond lengths from their mean value according to the equation D = (D′ + ΔD), where D′ is the length found in a polyhedron having equivalent bonds and ΔD is the bond distortion. For a given atom, D′ is expected to be similar from one structure to another, whereas D should vary as a function of ΔD. However, in several crystal structures D significantly varies without any relevant contribution from ΔD. In accordance with bond-valence theory, D variation is described here by a new equation: D = (D RU + ΔD top + ΔD iso + ΔD aniso + ΔD elec), where D RU is a constant related to the type of cation and coordination environment, ΔD top is the topological distortion related to the way the atoms are linked, ΔD iso is an isotropic effect of compression (or stretching) in the bonds produced by steric strain and represents the same increase (or decrease) in all the bond lengths in the coordination sphere, ΔD aniso is the distortion produced by compression and stretching of bonds in the same coordination sphere, ΔD elec is the distortion produced by electronic effects. If present, ΔD elec can be combined with ΔD aniso because they lead to the same kind of distortions in line with the distortion theorem. Each D-index, in the new equation, corresponds to an algebraic expression containing experimental and theoretical bond valences. On the basis of this study, the ΔD index defined in bond valence theory is a result of both the bond topology and the distortion theorem (ΔD = ΔD top + ΔD aniso + ΔD elec), and D′ is a result of the compression, or stretching, of bonds (D′ = D RU + ΔD iso). The deficiencies present in the bond-valence theory in explaining mean bond-length variations can therefore be overcome, and the observed variations of D in crystal structures can be described by a self-consistent model. © 2014 International Union of Crystallography
Bond-valence constraints around the O1 site of tourmaline
No full textThe stabilities of possible (Y)(R3+ + R2+ + Li+) clusters around the W anion (O1 site) of the tourmaline structure were checked using the bond-valence approach. Arrangements involving R3+ = Al3+ or Fe3+ and R2+ = (Fe, Mn, Mg)(2+) were all found to be stable. Structural data show a strong linear correlation between the mean formal valence (MFV) of the Y cations and the long-range average bond valence sum (BVS) at the O1 site, as estimated from bond-valence parameters. This correlation is observed for all chemical compositions of tourmaline, except for fluor-buergerite where the O3 site is dominated by oxygen anions. Results show that the long-range site populations of the Y and O1 sites are related to each other by valence constraints described by the empirical and theoretical equations: BVS(O1) = [0.99 MFV(Y) - 1.20] and MFV(O1) = [1.00 MFV(Y) - 1.00], respectively. The systematic deviation of the empirical equation from the ideal one is ascribed to the occurrence of bond strain involving the O1 site. An important implication of the correlation between MFV(Y) and BVS(O1) is that the (OH) content at the O1 site may be estimated by the equation (W)(OH) = 2 [1.01 BVS(01)] - 0.21 - F
Disordering of Fe2+ over octahedrally coordinated sites of tourmaline
No full textThe partitioning of iron among octahedrally coordinated sites in tourmaline, and its stereochemical consequences, were investigated in a Fe-rich dravite in a skam rock from Utö, Sweden. A multi-analytical approach using structure refinement (SREF), electron microprobe analysis (EMPA), and Mössbauer spectroscopy (MS) established the chemical and structural nature of the tourmaline. A structural formula obtained by optimization procedures indicates disordering of Al, Mg, and Fe2+ over the Y and Z sites, and ordering of Fe3+ at the Y site. Two Fe-rich tourmalines from the literature, re-examined with the optimizing site assignment procedure, appear to have iron partitioning comparable to that of the Utö tourmaline with Fe2+ disordered over the octahedral sites. This is best explained by disordered Fe2+ distributions that minimize the strain state of the Y-O bonds and provide a shielding effect reducing Y-Z repulsion. This is consistent with predictions from bond-valence theory and Pauling's rules. An indication of Z-site occupancy by Fe2+ in tourmaline may be signaled by a significant correlation between <Z-O> and the c lattice parameter (r2 = 0.96). The c value for a very Fe2+-rich tourmaline and an ideal end-member schorl, with Fe2+ and Al ordered at Y and Z (respectively), yielded <Z-O> values larger than 1.907 Å (the likely bond length for < ZAl-O>). These large <Z-O> lengths indicate that Fe2+ occurs at the Z site. The hypothesis of a dragging effect from <Y-O> to explain lengthening of < ZAl-O> is not supported by experimental evidence
Octahedrally coordinated vacancies in tourmaline: a theoretical approach
No full textBond-valence theory is used to explore the local arrangements involving vacancies at the Y and Z sites in the tourmaline structure. The local bond-valence requirements of all possible local arrangements around the O8, O7, O6, O3 and O1 anion-sites have been determined for Y- and Z-site vacancies locally associated with Li1+, Mg2+, Al3+, Fe2+, Fe3+, B3+ and Si4+. The results show that arrangements involving tetrahedrally coordinated R-T(3+)-cations and octahedrally coordinated R-Y(2+)- and R-Z(2+)-cations around O8, O7 and O6 can be ruled out, together with arrangements involving vacancies and Li-Y(1+). As the occurrence of a Y-site vacancy ((Y)square) is more in accord with the valence-sum rule than the occurrence of a Z-site vacancy ((Z)square), (Y)square is more likely to occur than (Z)square in tourmaline. Local arrangements involving vacancies around O1- and O3-sites can be stable for OH-, but not for O2-. Of particular interest in this regard are the arrangements [R-Y(3+) R-Y(3+) (Y)square]- (O1)(OH-) and [R-Z(3+) R-Z(3+) (Y)square]-(O3)(OH-), which yield the smallest deviations from the valence-sum rule. Coupling these stable arrangements with 2 x [Si-T(4+) R-Z(3+) (Y)square]-(O6)(O2-) forms larger vacancy clusters: [(Y)(R3+)(2)-(O1)(OH-)-(Y)(square)-(O3)(OH-)-(O6)(O2-)(2)-(R-Z(3+) Si-T(4+))(2)]. In tourmaline, vacancies are more favoured to occur at Y rather than at Z, in tandem with OH- at O3 and O1, R3+ at Y and Z and Si4+ at T
STEREOCHEMICAL CONSTRAINTS IN TOURMALINE: FROM A SHORT-RANGE TO A LONG-RANGE STRUCTURE
No full textExtension of the local bond-valence approach from Mg-Al to Fe(2+)-Fe(3+) short-range arrangements is explored in the structure tourmaline. Stable local arrangements involving trivalent (R(3+)) cations (Al, Fe(3+)) and divalent (R(2+)) cations (Mg, Fe(2+)) around the W and V anion sites in Li-free tourmaline result from short-range bond-valence requirements. The coupling of these stable local arrangements determines the formation of larger clusters of octahedra of general form [WY(3)VZ(2)], which can have either ordered or disordered distributions of R3+ and R2+ cations. These clusters are related through four different expressions: 1) 2 (Y)R(2+) + (Z)R(3+) + (W)(OH)(1-) reversible arrow 2 (Y)R(3+) + (Z)R(2+) + (W)O(2-), 2) 2 (Y)R(2+) + 2 (Z)R(3+) + (W)(OH)(1-) reversible arrow 2 (Y)R(3+) + 2 (Z)R(2+) + (W)O(2-), 3) (Y)R(2+) + 2 (Z)R(3+) reversible arrow (Y)R(3+) + 2 (Z)R(2+), and 4) (Y)R(2+) + (Z)R(3+) reversible arrow (Y)R(3+) + (Z)R(2+). Such relations describe the occurrence of both R(3+) cations at the octahedrally coordinated Y site and R(2+) cations at the octahedrally coordinated Z site of tourmaline, and lead to long-range ordered or disordered arrangements. In nature, disordered structural formulae are the rule owing to long-range requirements of geometrical fit and the minimization of strain
Tourmaline crystal chemistry
No full textTourmalines form the most important boron rock-forming minerals on Earth. They belong to the
cyclosilicates with a structure that may be regarded as a three-dimensional framework of octahedra ZO6
that encompass columns of structural “islands” made of XO9, YO6, BO3, and TO4 polyhedra. The overall
structure of tourmaline is a result of short-range and long-range constraints resulting, respectively on the
charge and size of ions. In this study, published data are reviewed and analyzed to achieve a synthesis
of relevant experimental results and to construct a crystal-chemical model for describing tourmalines
and their compositional miscibility over different length scales. Order-disorder substitution reactions
involving cations and anions are controlled by short-range structural constraints, whereas order-disorder
intracrystalline reaction involving only cations are controlled by long-range structural constraints. The
chemical affinity of a certain cation to a specific structural site of the tourmaline structure has been
established on the basis of structural data and crystal-chemical considerations. This has direct implications
for the tourmaline nomenclature, as well as on petrogenetic and provenance information. Some
assumptions behind the classification scheme of tourmaline have been reformulated, revealing major
agreement and significant improvements compared to earlier proposed scheme
Gatedalite, Zr(Mn2+2Mn3+4)SiO12, a new mineral species of the braunite group from Långban, Sweden
No full textGatedalite, Zr(Mn2+2Mn3+4)SiO12, is a new mineral of the braunite group. It is found in hausmannite-impregnated skarn together with jacobsite, Mn-bearing calcite, tephroite, Mn-bearing phlogopite, långbanite, pinakiolite and oxyplumboroméite at the Långban Mn-Fe oxide deposit, Värmland, central Sweden. The mineral occurs as very rare, small (≤60 μm), grey, submetallic, irregularly rounded anhedral grains. Gatedalite has a calculated density of 4.783 g/cm3. It is opaque and weakly anisotropic with reflectivity in air varying between 17.1 and 20.8% in the visible spectral range. Gatedalite is tetragonal, space group I41/acd, with the unit-cell parameters a = 9.4668(6) Å, c = 18.8701(14) Å, V = 1691.1(2) Å3 and Z = 8. The crystal structure was refined to an R1 index of 5.09% using 1339 unique reflections collected with MoKα X-ray radiation. The five strongest powder X-ray diffraction lines [d in Å, (I), (hkl)] are: 2.730(100)(224), 2.367(12)(040), 1.6735(12)(440), 1.6707(29)(048) and 1.4267(16)(264). Electron microprobe analyses in combination with single-crystal structure refinement resulted in the empirical formula: (Zr4+0.49Mn2+0.40Mg0.07Ca0.02Zn0.01Ce3+0.01)Σ1.00(Mn3+4.44Fe3+0.59Mn2+0.57Mg0.41Al0.01)Σ6.02Si0.99O12. Gatedalite is a member of the braunite group (general formula AB6SiO12). It is related to braunite (Mn2+Mn3+6SiO12) through the net cation exchange (Zr4+ + Mn2+) → 2Mn3+, which results from the substitutions Zr4+ → Mn2+ at the 8-fold coordinated site (A in the general formula) coupled with a 2Mn2+ → 2Mn3+ substitution at the 6-fold coordinated sites (B in the general formula)
Cation ordering in Pb2+-bearing, Mn3+-rich pargasite from Langban, Sweden
No full textA multi-analytical approach using electron microprobe analysis, X-ray structural refinement, and polarized single-crystal Fourier transform infrared spectroscopy was used to characterize short-range and long-range structures of a Pb2+-bearing, Mn3+-rich pargasite. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, demonstrate that Pb2+ is strongly ordered at the A-site in the monoclinic C2/m amphibole structure. This finding is in agreement with the observed ordering of Pb2+ in the rare Pb2+-bearing P2/a amphibole joesmithite, but is in contrast to a Pb2+ preference for the M4 site suggested by some studies on element partitioning between C2/m amphiboles and melts. Mn3+ is strongly ordered at the M2 site in structure of the present amphibole. Contrasting results obtained for mean M2-O bond lengths and reported local Mn3+-O bond lengths as well as between bond-length distortion of the mean M2 polyhedron and the local Mn3+-centered M2O(6) octahedron in pargasite indicates that the structural relaxation of this polyhedron is complete or nearly so
Oxyplumboroméite, Pb2Sb2O7, a new mineral species of the pyrochlore supergroup from Harstigen mine, Värmland, Sweden
No full textOxyplumboroméite, Pb2Sb2O7, is a new mineral of the roméite group of the pyrochlore supergroup (IMA 2013-042). It is found together with calcite and leucophoenicite in fissure fillings in tephroite skarn at the Harstigen mine, Värmland, Sweden. The mineral occurs as yellow to brownish yellow rounded grains or imperfect octahedra. Oxyplumboroméite has a Mohs hardness of ~5, a calculated density of 6.732 g/cm3 and is isotropic with a calculated refractive index of 2.061. Oxyplumboroméite is cubic, space group Fd3̄m, with the unit-cell parameters a = 10.3783(6) Å, V = 1117.84(11) Å3 and Z = 8. The strongest five X-ray powder-diffraction lines [d in Å (I)(hkl)] are: 2.9915(100)(222), 2.5928(32)(400), 1.8332(48)(440), 1.5638(38)(622) and 1.1900(12)(662). The crystal structure of oxyplumboroméite was refined to an R1 index of 3.02% using 160 unique reflections collected with MoKα radiation. Electron microprobe analyses in combination with crystal-structure refinement, infrared, Mössbauer and electronic absorption spectroscopy resulted in the empirical formula A(Pb0.92Ca0.87Mn0.09Sr 0.01Na0.05)Σ1.93 B(Sb 1.73Fe3+ 0.27)S2.00 X+Y[O6.64(OH)0.03]Σ6.67. Oxyplumboroméite is the Pb analogue of oxycalcioroméite, ideally Ca2Sb2O7. © 2013 The Mineralogical Society
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